StringBuilder vs String ?

[Pirate]

I'm using stringbuilder instead string (for critical performance issue) . Anyway , in this for loop ,
am I creating string objects(with the bolded font each time I call this line :

SplitedFilWriteStream = File.Create(Path.Combine(OutputPath ,FileNameOnly+segName[nSegmentCount-1].ToString() ));

The for loop looks like this :

for (nSegmentCount =1; nSegmentCount <=nSegments_CompletetBuffered;nSegmentCount++)
    {
segName.Append(nSegmentCount);    
SplitedFilWriteStream = File.Create(Path.Combine(OutputPath ,FileNameOnly+segName[nSegmentCount-1].ToString()));
SegmentTotal += 1;